Note that the percentage complete is 0% and that submitted, succeeded, errors, skipped and queued/processing are no longer changing:
02/20/2019 12:47:42 Native email message conversion: submitted=6, succeeded=0, errors=0, skipped=0, queued/processing=6, processTime(ms)=[low=0, high=0, average=0]
02/20/2019 12:47:42 Native loose file/attachment/embeddings conversion: submitted=2, succeeded=0, errors=0, skipped=0, queued/processing=2, processTime(ms)=[low=0, high=0, average=0]
02/20/2019 12:47:42 Html file conversion: submitted=0, succeeded=0, errors=0, skipped=0, queued/processing=0, processTime(ms)=[low=0, high=0, average=0]
02/20/2019 12:47:42 0% complete
02/20/2019 12:52:42 Native email message conversion: submitted=6, succeeded=0, errors=0, skipped=0, queued/processing=6, processTime(ms)=[low=0, high=0, average=0]
02/20/2019 12:52:42 Native loose file/attachment/embeddings conversion: submitted=2, succeeded=0, errors=0, skipped=0, queued/processing=2, processTime(ms)=[low=0, high=0, average=0]
02/20/2019 12:52:42 Html file conversion: submitted=0, succeeded=0, errors=0, skipped=0, queued/processing=0, processTime(ms)=[low=0, high=0, average=0]
02/20/2019 12:52:42 0% complete
This indicates that caching is now hung.
In addition to the above, checking the D:\CWShared\IGC\Jobs folder for a given server may show uncompleted jobs. See the figure below:

The eDiscovery Platform (eDP) uses two products to handle image rendering: Muhimbi and Information Graphics Corp (IGC). This issue is caused by eDP sending a job to the IGC queue and IGC failing to respond to the job.